Rising CO2 concentrations reduce nitrogen availability in alpine grasslands

Abstract

<p>Alpine grasslands, local biodiversity hotspots with very high nature conservation and cultural value, belong to the most affected by global change ecosystems. Yet, the potential effects of others than global warming factors on alpine plant functioning is poorly understood. To address this gap, we made use of 360 herbarium specimens from nine vascular plant species collected in the Bavarian Alps, Germany, extending back 200 years to reconstruct historical changes in foliar N content and stable isotope composition (δ<sup>15</sup>N), indicators of plant response to long-term N atmospheric deposition and rising atmospheric CO<sub>2</sub> concentrations ([CO<sub>2</sub>]). These changes were interpreted in terms of three competing hypothesis (eutrophication, oligotrophication and photorespiration), representing alternative explanations for the response of plants to changes of N and CO<sub>2</sub> availability.</p><p>Foliar δ<sup>15</sup>N decreased significantly over time but an explanation by an increased input of reactive N from long-distance transport (’eutrophication’ hypothesis) was unlikely because foliar N contents decreased significantly as well. An increased carbon gain due to increasing [CO<sub>2</sub>] (‘oligotrophication’) also was unlikely because instantaneous water use efficiency remained unchanged and indicated no increase in C gain. The detected patterns agreed well with the ‘photorespiration’ hypothesis that biochemically links N assimilation and carbon assimilation. Increasing concentration of ambient CO<sub>2</sub> that decreases photorespiration explained decreasing δ<sup>15</sup>N values (R<sup>2</sup> = 0.84, p < 0.001) and decreasing N contents (R<sup>2</sup> = 0.40, p < 0.036).</p><p>Our results suggest that increasing [CO<sub>2</sub>] by suppressing photorespiration reduces N availability to alpine plants. These findings contradict the generally accepted assumption of negative effects of eutrophication on alpine grasslands caused by air-borne N deposition. We conclude that increasing [CO<sub>2</sub>] should be considered as alternative driver of long-term changes in alpine ecosystems, as it affects directly the plant C:N stoichiometry, a key plant trait determining several important ecosystem processes.</p>